It is known that, if exposed to an undesirable environment, the performance of a semiconductor photodetector can become degraded even though sensitive surface areas of the photodetector, particularly where an electric field is present during operation of the photodetector in the vicinity of a p-n junction or a junction between semiconductor material and metal, are protected from the ambient by a deposited dielectric layer. For this reason a number of applications for photodetectors have required such photodetectors to be hermetically packaged in order to reduce the risk of occurrence of this kind of performance degradation. The packaging must include provision, both for electrical connections with the photodetector from outside the package, and for the feeding into the package of the optical signal that the photodetector is designed to detect. One way of effecting such provision is to provide a window in the package wall, as for instance described in GB 2 208 944A. If the photodetector is to receive light emitted from the end of an optical fiber, the construction of the package may be such as to make the minimum separation between the end of the fiber and the photodetector so great as to require some form of lens coupling between them. This may be provided by a lens within the package, by a lens outside the package, or by using a lens as part of the package window, as described in GB 2 208 944A. As an alternative to the use of a window, a wall of the package may be penetrated in a hermetically sealed manner by some form of light guide, as for instance described in GB 2 064 862A. In the case of the particular structure described in GB 2 064 862A, the outer end of the light guide is butted against the optical fiber, but a clearly valid alternative is to replace the light guide with an optical fiber having properties similar to, or identical with, those of the optical fiber whose output it is desired shall be detected by the photodetector.
The use of the optical fiber/guide hermetic feedthrough, by definition requires an hermetic seal around the optical fiber/guide, and this is typically expensive to manufacture and difficult to test for leaks because the fiber is usually provided outside the enclosure with mechanical protection in the form of a plastics protective coating. The use of a window in place of the hermetic fiber-guide feedthrough avoids these particular problems, albeit at the expense of adding significantly to the complexity and expense of the provision of a mechanical link between the photodetector chip and the optical fiber whose output is to be detected by the chip.